I don't believe that anyone has produced a definitive list of which 600X models support SpeedStep CPUs.
Originally, the belief was that you could just check to see which CPU your 600X originally shipped with: if it shipped with a PIII 650MHz, then it supported SpeedStep, otherwise it didn't. However, there have been scattered reports of other models appearing to support SpeedStep CPUs even though they originally shipped with a PIII 500MHz.
It is safe to say that ANY 600X that originall shipped with a PIII 650MHz will support SpeedStep, since the PIII 650MHz was only ever released as a SpeedStep CPU in the MMC-2 form factor. This includes:
2645-5Fx
2645-5GU
2645-5HU
2645-9WU
2645-9ZU
2645-9Fx
as well as possibly others.
The following models shipped with a PIII 500MHz, and I am not sure how to determine whether or not these models will support SpeedStep without actually inserting a SpeedStep CPU and giving it a try:
2645-3Ex
2645-4Ex
2645-5Ex
2645-7Ex
2645-8Ex
2645-9Ex
The following model originally shipped with a PIII 500MHz, but for some reason it appears separate from the other PIII 500MHz models and alongside the PIII 650MHz models in the IBM/Lenovo documentation, and some people believe that it therefore shipped with a motherboard that supports SpeedStep:
2645-8Px
But as I said at the outset, I don't think that these lists can be used to get a definitive answer on SpeedStep compatibility.
For some additional information, check out:
ThinkPad 600X System Boards
and
600X Motherboard
Phil.
